Ingestion, Digestion, Absorption

Absorption
Movement of monomers, vitamins,
minerals & H2O into lumen of
digestive tube
What happens to food?
• Three processes
– Digestion
– Absorption: the process of moving small molecules
across cell membrane and into cells
– Elimination
• Where does absorption occur?
– GI tract
Overview of Digestion
Digest these
Absorb these
Absorption occurs along entire
GI tract
• Stomach: little water, mostly drugs & some
vitamins
• Small Intestine: Absorption of everything occurs
here.
• Large intestine: mostly water, some minerals
and small fats
Specializations encourage absorption
• Increased SA (primarily
in jejunum)
– Plicae circulares: Major
folds of intestinal wall.
– Villi: Minor folds in close
contact with nutrients
– The brush border is
composed of microvilli
Regional Specialization
• Duodenum
– Few plicae circulares
– Short villi
• Jejunum
– Many plicae circulares
– Tall villi
– Many lacteals
Villi & microvilli
How is absorption achieved?
Outside
cell
Carrier loads
nutrient on
outside of cell . . .
Cell
membrane
Inside cell
SIMPLE DIFFUSION
Small nutrients (water &
small lipids) are
absorbed by simple
diffusion. Cross into
intestinal cells freely.
FACILITATED
DIFFUSION
. . . and then
releases it on
inside of cell.
Some nutrients (water-soluble
vitamins) are absorbed by facilitated
diffusion. They need a specific
carrier to transport them from one
side of the cell membrane to the
other.
Carrier loads
nutrient on
outside of cell . . .
. . . and then
releases it on
inside of cell.
ACTIVE TRANSPORT
Some nutrients (glucose &
amino acids) must be
absorbed actively. These
nutrients move against a
concentration gradient,
which requires energy.
Fig. 3-9, p. 81
Carbohydrate absorption
• Cotransport into
cells (indirect
active transport)
• Facilitated
diffusion into
capillaries
Carbohydrate Absorption
• Monosaccharides enter capillaries within intestinal villi.
In the liver,
galactose &
fructose are
converted to
glucose.
SI
Monosaccharides
travel to the liver via
the bloodstream.
Small intestine
Stomach
Fat Absorption
Monoglyceride
Short-chain
fatty acids
Medium-chain
fatty acids
Glycerol
Chylomicrons
Micelle
Protein
Triglyceride
Lacteal
(lymph)
Capillary network
Chylomicron
Blood vessels
To blood
To liver
Longchain
fatty
acids
Protein absorption
• AA & short
peptides absorbed
via cotransport
• Active transport
into capillaries
• Off to the liver
Vitamins & minerals
• Absorbed in small and large intestine
– Dissolved in lipids
– Via carrier proteins
– Via diffusion through membrane channels
Large intestine: colon + rectum
• Undigested food passes illeocecal valve (sphincter) to large
intestine
• Mostly “indigestible” polysaccharadies, bacteria and H20 remain
– Bacteria continue to digest fiber (byproducts =short chain fatty acids +
vitamins K & B12)
• Absorption of H2O & some nutrients
• Elimination
Colon histology
• Many goblet
cells
• Few absorptive
cells
Destination of nutrients
• Some lipids and lipid soluble vitamins enter
lymph system; This empties into bloodstream
• All others enter bloodstream directly
• ALL nutrients (save fats) travel to liver first (via
portal vein), which stores, processes &
regulates their release
• Liver filters ALL your blood
Head and
upper
body
Nutrient delivery
Lungs
Pulmonary vein
Pulmonary artery
Aorta
Left
side
Right
side
Heart
Hepatic artery
Hepatic vein
Portal vein
Liver
Digestive
tract
Lymph
Entire body
= Arteries
= Capillaries
= Veins
= Lymph vessels